Microfluidics is a technology dealing with diminutive amounts of flowing liquid solutions, which are fed through microchannels placed on microchips. Said technology is rapidly emerging as a new, more sensitive alternative to the powerful oligomer-chip technology.
The microfluidic systems have been used for purification, separation or sequencing and include methods such as microcapillary electrophoresis, packed bed immuno- or enzyme-reactors (U.S. Publ. Appl. No. 2002/0023841, U.S. Publ. Appl. No. 2004/0094419, PCT Publ. Appl. No. WO 2005/09481, PCT Publ. Appl. No. WO 03/099438, and PCT Publ. Appl. No. WO 2007/035498). Microfludic devices with pillar filters are described in PCT Publ. Appl. No. WO 2008/024070, PCT Publ. Appl. No. WO 01/85341, PCT Publ. Appl. No. WO 2007/098027, PCT Publ. Appl. No. WO 99/09042, and PCT Publ. Appl. No. WO 02/093125, as well as in Liu et al., Electrophoresis, November 2007, vol. 28, 4173-4722), but automation and miniaturizing of binding assays are also suggested. Conventional binding assays usually take place in solution and include reactions between binding partners and their counterparts which together form binding pairs. Examples of binding pairs are antibodies and antigens or complementary probe and target sequences. In a typical binding assay the binding partners of the binding pairs are alternating between solid and liquid phases with intermediate purification and extraction stages, which are performed on microbeads.
In the patent literature, few of the problems encountered in miniaturizing conventional binding assays are discussed, but it is evident that magnetic particles, which are very convenient in macroscale conventional binding assays, are not quite as easy to manipulate when used in microfluidic applications. This is probably a reason why magnetic microbeads have been used mainly for concentration and isolation by retaining them within certain regions of microchannels having a diameter smaller than that of the microbead. Microfluidic pillars have also been used in microfluidic channel systems as mechanical stoppers of microbeads. The adsorption and desorption reactions between the partners of the binding pairs as well as the purification stages, require application of thorough and efficient mixing systems in order to allow sufficient contact between the target binding partners in the sample and their counterparts on the surface of microbeads or vice versa. Therefore, in prior art, the adsorption/desorption steps and purification steps are generally carried out before feeding the liquid stream with processed target binding partners into the microfluidic channel system for subsequent separation and detection. In order to obtain adequate mixing in microfluidic systems the application of physical forces, such as acoustic forces have been suggested, but methods particularly aiming at manipulation of magnetic microbeads in the microfluidic channels are not suggested.
Gas bubble generation caused by electrical fields in aqueous solutions is discussed in U.S. Publ. Appl. No. 2006/0228749, and various physical forces are suggested for handling the problem, but the fact that bubble formation is a frequently encountered difficulty whenever a liquid stream is fed into a microfluidic channel system is not discussed, even if air bubbles in a microscale system, where the volume of a bubble is very big as compared to the volume of the liquids fed into the system, is a problem that can seriously distort any results obtained by using microfluidic methods.